Field of the Invention
The present disclosure relates to control of an image forming apparatus, particularly to control of an electrophotographic image forming apparatus.
Description of the Related Art
An electrophotographic image forming apparatus is in widespread use. The electrophotographic image forming apparatus in a printing process executes a step of forming a toner image corresponding to an input image on an image carrier such as a photoreceptor and an intermediate transfer body, a step of transferring the toner image on the image carrier to a sheet of paper, and a step of fixing the toner image to the paper.
An alternating current (AC) transfer technique is known as a technique of transferring the toner image from the image carrier to the paper. In an AC transfer process, an AC voltage which is opposite in polarity to the toner image and on which a DC component and an AC component are superimposed is applied to the toner image by a transfer member. The AC voltage applied in the AC transfer process will also be hereinafter referred to as a “transfer voltage”. The application of the transfer voltage causes a reciprocating motion of toners, among which physical and electrical interactions occur. As a result, adhesion of the toners is reduced to result in improved transferability of the toner image from the image carrier to the paper. Unevenness in density of the toner image is prevented as well.
However, the application of the transfer voltage sometimes causes the paper to be electrically charged and attracted to the image carrier. An AC charge removing technique that separates the paper from the image carrier is known as a technique of preventing such situation. In an AC charge removing process, an AC voltage which is opposite in polarity to the transfer voltage and on which a DC component and an AC component are superimposed is applied to the paper by a charge removing member. The electrical charge is removed from the paper as a result. The AC voltage applied in the AC charge removing process will also be hereinafter referred to as a “charge removing voltage”.
Regarding a technique in which the AC transfer technique and the AC charge removing technique are combined, JP 61-87179 A discloses a copying machine “capable of performing stable transfer and separation without causing retransferring”. JP 11-38783 A discloses an image forming apparatus that “smoothly removes an electrical charge accumulated in an intermediate transfer body”. JP 2013-83951 A discloses an image forming apparatus that “prevents a leak between members to which AC is applied”.
The copying machine disclosed in JP 61-87179 A detects an image region from an original and applies a lower DC component voltage of the AC voltage to the image region than that applied to a non-image region. However, the electrical charge is not effectively removed from the paper when the DC component voltage is decreased.
The image forming apparatus disclosed in JP 11-38783 A applies the charge removing voltage opposite in phase to the transfer voltage to the intermediate transfer body in the AC charge removing process. The image forming apparatus thus removes the electrical charge from the intermediate transfer body. However, the phase of the transfer voltage and the charge removing voltage is shifted from what is intended in some cases when the paper is not stably conveyed, for example. In this case, the image forming apparatus disclosed in JP 11-38783 A cannot effectively remove the electrical charge from the intermediate transfer body.
A potential difference between the transfer member and the charge removing member is increased when the AC transfer technique is combined with the AC charge removing technique, in which case there is a possibility that an electrical discharge (leak) occurs between the transfer member and the charge removing member. In order to prevent such leak, the image forming apparatus disclosed in JP 2013-83951 A sets the transfer voltage and the charge removing voltage in phase with each other. This allows the potential difference between the transfer member and the charge removing member to be decreased to prevent the leak therebetween. However, when the transfer voltage and the charge removing voltage are set to be in phase with each other, a potential difference between the paper and the charge removing member is fixed so that the electrical charge is not effectively removed from the paper. As a result, unevenness in the potential of the paper is not eliminated to cause a failure to convey the paper and noise accompanying the failure. Moreover, the unevenness in the potential possibly affects the toner image and degrades print quality.